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Over the last few decades physicists have been developing a curious idea. Perhaps the world we inhabit is a hologram, lacking a crucial feature of the world as we perceive it: the third dimension.
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At the beginning of an epidemic the number of infected people grows exponentially. But why does the number e appear in descriptions of this growth?

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The doubling time of a disease is the time it takes for the number of cases of the disease to double. How do you calculate it?